This invention relates to wireless telecommunication equipment, and in particular, to the mobile station of a code division multiple access (CDMA) cellular phone system. The CDMA system, which employs spread spectrum modulation, has a base station and a plurality of mobile stations wherein each mobile station includes a receiver portion and a transmitter portion, and is provided with a transmission power controller.
In the CDMA system, each mobile station is provided with a spread spectrum demodulator which despreads a received spread spectrum signal in accordance with a distinctive spreading code assigned to that mobile station. Each mobile station has a different spreading code, and interference often occurs between the mobile stations. The amount of interference depends on the correlation characteristic of the spreading code of each mobile station.
As the interference occurring when a signal is sent from one mobile station to another increases, the signal-to-noise ratio (S/N) of the demodulated signal decreases. Consequently, the strength of the radio wave transmitted by the mobile station must be constantly changed as a function of the distance between the mobile station and the base station, and to compensate for fading due to variations in the transmission path. This means that to maximize system capacity it is necessary to control transmission power so that the radio signal transmitted from each mobile station to the base station remains at the same level.
The transmission power of a conventional CDMA mobile station is controlled by employing both open and closed loop control of the power transmitted on a down channel from the base station to a mobile station. In the open loop power control, the control circuits of the mobile station initially assume that the radio signal is attenuated in the same way in both the down channel and in the up channel from the mobile station to the base station. An estimate is made of the propagation loss in the down channel by measuring the received power level at the mobile station, predicting the power required for the up channel and using this information to control the power transmitted by the mobile station.
FIG. 2 is a graph showing the open loop power control characteristic of a CDMA cellular portable telephone which complies with the United States standard for CDMA telephones. In FIG. 2, the transmission power of a mobile station is plotted against the strength of the signal received at the mobile station and shows that, using open loop power control, the dynamic range of the transmission power must be at least 80 dBm. However, a closed loop power control is also needed because the propagation losses of the down and up channels are not the same since each channel uses a different frequency band.
In the closed loop power control, the base station measures the received signal strength, and sends a power control bit to a mobile station along with normal information data at 1.25 ms intervals using a down channel. The mobile station provides a fine control of the transmitted power with a 1 dB pitch, which is in addition to the transmitted power predicted by the open loop power control. That is, the open loop power control provides an approximate or rough control of the transmitted power, and the closed loop power control performs a fine control of the transmitted power. However, the characteristic of the conventional open loop power control is such that the closed loop power control is nDt sufficiently effective. Thus, there is a need for an improved open loop power control.
The open loop power control characteristic of the conventional CDMA cellular portable telephone standardized for use in the United States has an allowable error in the transmitted power strength which is within .+-.9.5 dBm despite variations in the environment including changes in temperature. This is in comparison with the received power (receive signal strength) vs. transmission power (transmission signal strength) characteristic shown in FIG. 2.
However, the receiver and transmitter portions of conventional mobile stations are comprised of power amplifiers and many other functional components having individual gain variation characteristics. These variations, added to variations in the environmental temperature can result in the allowable error being exceeded if the differences in gain variation between components of the receiver and transmitter portions of the mobile station are unduly large. In other words, using a conventional open loop power control wherein the transmission signal strength in the transmitter portion of the mobile station is controlled in accordance with the strength of the received signal, the allowable error at a mobile station could be exceeded if the difference in the gain variations between the receiver and transmitter portions of the station is too large. This problem occurs not only in mobile stations associated with CDMA cellular portable telephone systems but also in other wireless telecommunication equipment having open loop power control.